Electromagnetic force plays a main role relating to the great majority of chemical, physical and biological phenomenon in a wide range of material realm from atom to macroscopical objects except the involvements in the domains of atomic nucleus, elementary particles and astrophysics. The physical and chemical properties of the materials are also basically decided by the electromagnetic force. The electromagnetic force is very mighty. In almost all of natural phenomena, electromagnetic force plays an important even decisive role. The basic force of atomic nucleus combining electron to constitute atom, atoms joining together to compose molecules, molecules combining each other to form macroscopic objects (liquid, solid et al), is the electromagnetic interaction.
Electromagnetic force is carried by photons. Electromagnetic radiation, such as light, can be thought of as being composed of photons. Photons of different energies (wavelength) span the electromagnetic spectrum of γ rays, x rays, UV, visible light, infrared, microwave, radio waves, and so forth. Uneven spatio-temporal distribution of energy is the impulse of nature self-organization, information sources of nature complexity and force of nature order. Macroscopical phenomenon of nature strongly depends upon her microscopic state of energy.
Direct or indirect cumulation and application of photon energy is the most important common character of all kinds of life on earth. The energy variation of life relates to low level energy photon, especially infrared radiation.
Since their invention in the late 1500s, light microscopes have improved our knowledge of physical and chemical sciences and clinical and biological research.
The domain of modern microscopy can be divided into three defined approaches: far-field, near-field and contact-field optics. Because of the interference of medium between sample and detector, it is difficult to monitor the dynamic variation of microscopic state of energy in nanometer range of sample by means of far-field and near-field optics.1. Far-Field and Near-Field Optics
Traditional optical lens equipped microscopes are based on far-field optics, where the distances between the sample and the detector are much larger than the wavelength of the light source. It is well known that the resolution of the far-field optical instruments is originally determined by E. Abbe barrier, it is not finer than λ/2, where λ is the wavelength of the light source. For visible light, it is only about 200 nm.
Unlike traditional microscopes, scanning probe microscopes are near-field microscopes such as scanning tunneling microscopy, atomic force microscopy and near-field scanning optical microscopy, the techniques to overcome the diffraction limit, do not use lenses, so the size of the probe rather than diffraction effects generally limit their resolution. All of these microscopes work by measuring a local property—such as tunneling current, height, optical absorption, or magnetism—with a probe or “tip” placed very close to the sample.
The revolutionary development of scanning probe microscopy has made the imaging of objects at molecular or atomic level possible. The near-field scanning optical microscope, one of the most recent entries in the burgeoning field of scanned probe microscopy, opened the possibilities of ultra-high optical resolution at the nanometer scale which overcomes the Rayleigh diffraction limit. Near-field scanning optical microscope obtains super-resolution optical images by use of a sub-wavelength optical probe. The probe consists of a tapered optical fiber which is coated with a thin opaque metal at the tapered end. The size of the aperture is the dominant factor in determining the resolution of a near-field optical imaging device.
A near-field probe has been developed that yields a resolution of about 12 nm (about lambda/43) and signals about 10,000—to one million-fold larger than those reported previously. With these probes, near-field microscopy appears poised to fulfill its promise by combining the power of optical characterization methods with nanometric spatial resolution, as described in an article by E. Betzig et al. entitled “Breaking the diffraction barrier—Optical microscopy on a nanometric scale,” in Science, vol. 251, Mar. 22, 1991, p. 1468–1470.
U.S. Pat. No. 5,633,972 describes a super resolution imaging fiber for subwavelength light energy generation and near-field optical microscopy. The imaging fiber comprises a unitary fiber optical array of fixed configuration and dimensions comprising typically from 1,000 to 100,000 optical fiber strands which terminate at one array end as tapered strand end faces limited in size to a range from about 2–1,000 nanometers in diameter. Overlying these tapered strand end faces is a thin opaque metal coating having a size-limited end aperture ranging from about 2 to less than about 1,000 nanometers in diameter. These size-limited end apertures provide for the generation of a plurality of discrete subwavelength light beams concurrently.
2. Some Contact-Field Optics Related Technologies
Fiber Optic
In recent years it has become apparent that fiber-optics are steadily replacing copper wire as an appropriate means of communication signal transmission. They span the long distances between local phone systems as well as providing the backbone for many network systems. Other system users include cable television services, university campuses, office buildings, industrial plants, and electric utility companies.
A fiber-optic system is similar to the copper wire system that fiber-optics is replacing. The difference is that fiber-optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down copper lines and fiber optic wire carries much more information than conventional copper wire and is far less subject to electromagnetic interference. Total internal refection confines light within optical fibers. Because the clad has a lower refractive index than that of the core, light rays reflect back into the core if they encounter the clad at a shallow angle. A ray that is less than a certain “critical” angle escapes from the fiber.
U.S. Pat. No. 6,801,698, issued Oct. 5, 2004 relates to high index-contrast fiber waveguides and the materials for forming high index-contrast fiber waveguides. The optical fibers that utilize total internal reflection to confine light to a core can provide enhanced radial confinement of an optical signal in the fiber waveguide. The enhanced radial confinement can reduce radiative losses, thereby improving transmission efficiency. Moreover, in addition to enhanced radial confinement, it is also possible to achieve enhanced axial confinement in the fiber waveguide. The fiber core has a refractive index more than 2.5 (such as 2.8) and the fiber cladding has a refractive index less than 1.45 (such as 1.4). The absolute difference between the refractive indices of the fiber core and cladding is more than 1.2 (such as 1.4).
Fiber Optical Taper
The taper is essentially an aligned bundle of a large number of optically transmissive fibers fused together to form a coherent bundle. Each of the component fibers of the aligned bundle is a filament and composed of a high-index-core material such as glass or quartz surrounded by a lower-index cladding. Only the cores transmit imaging light.
The bundle is heated in the center, stretched and cut into two tapers, forming a truncated cone shape, resulting in variation of its diameter from one end to the other. In this process each fiber is stretched and is tapered as well. When carried out under well-controlled conditions, the stretching process produces a taper having a minimum of image distortion. The diameter of each filament increases uniformly from the minor diameter end of the taper to the major diameter end. When such a fiber optical taper is placed with its small face in contact with an object (zero working distance), an enlarged image appears on the larger face because light from an element of the object field that enters the small end of a given fiber is trapped in the fiber by total internal reflection until it emerges from the large diameter face. The magnification of a taper is simply the ratio of the diameters of the end faces. The magnified image is real and appears at the top face.
In minification, light from an element of an object field enters the large diameter end of a fiber and is “funneled” down to the small diameter end. In either case, the light exiting from the fibers forms a planar image field corresponding to the planar geometry of the exit surface. Magnification over the image field is uniform since the diameters of each fiber end lying in each end surface are identical.
The numerical aperture (NA) of optical fibers and bundles of fibers is a measure of the angular width of the cone of light that is captured by the fibers. This parameter is measured by the maximum angle of obliquity at which an image is still observable on the face of the fiber optical bundle. Beyond this angle the image fades off. For a fiber of uniform diameter (nontapered), the nominal or intrinsic numerical aperture is determined by the glasses which comprise the fiber core and the cladding.It is given, for a fiber of uniform diameter, by:             N      .      A      .        =                            N          0                ⁢                                  ⁢        sin        ⁢                                  ⁢        α            =                                    N            1            2                    -                      N            2            2                                ,where N0 is the refractive index of the external medium (for air, N0=1); N1 is the refractive index of the core; and N2 is the refractive index of the cladding. The angle α is the half-angle of the cone of light captured or emitted by the fiber. This parameter is important in the use of a taper as a magnifier because it determines both the light-gathering capability and the angular field of viewing around the taper, from which the enlarged image on the top face of the taper is visible.Tapered fibers are governed by one important law,             d      1        ⁢    sin    ⁢                  ⁢          θ      1        =            d      2        ⁢                  ⁢    sin    ⁢                  ⁢          θ      2      where diameters d1, d2 and angles θ1, θ2 are shown in FIG. 1. d1 is the small core end diameter and d2 is the large core end diameter of the tapered optical fiber. θ1 is the incident light angle and θ2 is the light angle after several total internal reflections in the fiber core.
The angle of reflection of a light ray is equal to the angle of incidence; therefore, light entering the small end of a fiber becomes more collimated as the diameter increases because the reflecting surface is not parallel to the fiber axis.
In a fiber-optical taper the effective numerical aperture (N.A.large face) is determined by the tapering of the fibers. The reduction of N.A.large face in such fibers is inversely proportional to the magnification. In a cone-shaped fiber the following relations hold for the angles α and α′ at the two ends:             Sin      ⁢                          ⁢              α        ′              =          sin      ⁢                          ⁢              α        /        M              ,          ⁢            NA              large        ⁢                                  ⁢        face              =                  N        .        A        ⁢                  .                      small            ⁢                                                  ⁢            face                              /      M        ,where M is the magnification, α and α′ are the obliquity angles at the small and the large ends, respectively. In general, it is desirable for NAlarge face of the taper to be as large as possible. Obtaining a high nominal NA in turn requires the use of a core glass with a high refractive index, or a very-low-index cladding glass, or both.
Image resolution in fiber bundles is generally related to the fiber size. The fiber optic resolution is shown in FIG. 2. For static resolution (no scanning), a commonly used criterion for resolution is:             1      /      3        ⁢    d    <  R  <            1      /      2        ⁢    d  where d is the diameter of the fiber in millimeters and R is the resolution in line pairs.
Each fiber in the taper is composed of a high-index-core glass surrounded by a lower-index cladding. The light transmission of a fiber-optical taper is given in terms of (1) the internal transmittance of the core glass of the fibers, (2) the Fresnel reflection losses at the faces, and (3) the packing fraction (PF). The PF is the ratio of core area to the total taper face area (i.e., core plus cladding). The thinner the cladding relative to the core, the higher the PF. The P.F. of commonly used tapers is on the order of 50% to 75%. The non-imaging light transmitted by the cladding in turn limits the contrast transferred through the taper.Thus the transmission through the cores is given by:       T    =                  PFt        f            ⁢                          ⁢      exp      ⁢                          ⁢              (                              -                          β              λ                                ⁢          L                )              ,where tf is the Fresnel transmission factor, β80  is the absorption coefficient of the core glass, and L is the length of the taper.
U.S. Pat. No. 6,801,697, issued Oct. 5, 2004, describes a view fiber optic taper. The apparatus comprises a bundle of optic fibers having a base end and a viewing end. When the small end of a fiber optic taper is placed in contact with an object such as a printed page, an enlarged image appears at the upper, larger face of the taper. Specifically, the size of the transmitted image is in direct proportion to the change in size of the two ends of the fiber optic taper. Size ratios, i.e., magnifications, of from nearly unity to as much as 10:1 may be practically obtained using a fiber optic taper. Each fiber in the bundle transmits one “pixel” of light from an image at one end of the fiber optic taper to the other end of the taper.
U.S. Pat. No. 5,600,751, issued Feb. 4, 1997, relates to fiber optic reading magnifiers. A reading magnifier formed by a bundle of juxtaposed longitudinally tapered optical fibers having a viewing end and a flat base end. The viewing end of the optical fiber bundle is at the larger end. Their main advantage over other reading aids is their relative stability and ease of use. The fixed and stable distance from the reading material and the flexibility in distance from the eye to the magnifier make their use easy to learn.
Fiber-optic tapers do not exhibit any of the so-called Seidel aberrations of lenses, such as spherical aberration, chromatic aberration, coma, or astigmatism. When properly made, they exhibit no significant distortion.
Tapered image conduit is available in round, square, and hexagonal formats and can be fabricated in the form of almost any regularly shaped polygon. Tapers may be used both for the magnification and for the minification of objects.
FIG. 3 shows a square grid pattern can be easily magnified by a fiber optical taper without focusing.
FIG. 4 shows the structure of compound eye of the insects. The compound eye is the most common eye in the nature. It is an array of tapered fibers, each forming a narrow acceptance angle and each looking in a different direction.
Multiple Channel Ion Sensitive Micro-Electrodes
Multiple channel ion sensitive micro-electrodes have a tapered glass capillary structure produced by heating and pulling technologies to generate very tiny tapered glass tip with nano-meter diameter opening. The structure of two channel ion sensitive microelectrode is shown in FIG. 5.
Ion sensitive microelectrodes provide a means of directly assessing the living cell extra cellular or intracellular activities of organic or inorganic ions and for making prolonged measurements of these without great damage to the living cell. The advantage of the ion-sensitive microelectrode is its very high spatial resolution. Multiple channel ion sensitive microelectrode is a good example to demonstrate how high the spatial resolution of a multiple channel chemical sensor could be. The tip outer diameter of two barrel ion-sensitive microelectrode can be made about 0.1 um. Each channel inner diameter of the chemical sensitive area at the tip opening of the microelectrode may be as small as 20 nm.
Several important neuro-transmitters (acetylcholine, serotonin and histamine) and bile acid ion sensitive microelectrodes have been described in the following references:    Bi Yu, Chinese Patent 87104761, “The liquid ion exchanger compounds for acetylcholine ion sensitive microelectrode”, issued on May 1990.    Bi Yu, et al. “Miniaturization of a liquid membrane sensor for the determination of bile acids,” Biosensors & Bioelectronics 5 (1990) 215–222.    Bi Yu, “Histamine selective microelectrode based on synthetic organic liquid ion exchanger,” Biosensors 1989, 4, 373–380.    Bi Yu, “A newly developed ion selective microelectrode suitable for determination of serotonin.” Chinese J. Physiological Science 1989, 5 (1), 10–17.
In the U.S. Pat. No. 6,396,966, Lewis, et al. described a glass structures for nanodelivery and nanosensing. The techniques described in this patent can produce high efficiency throughput of light through tapered glass structures with subwavelength apertures at the tip are unique and permit the generation of ultra small spots of light that could extend to below 20 nm with significant intensities of light in these tips. In addition with small amendations these structures can be altered for excellent delivery of nanoquantities of chemicals with nanometric control of chemistry using the force sensing capabilities of these structures and other uses such as combined force and ion sensing etc.
Capillary Lens
It is well known that hollow glass capillaries can act as waveguides for x-rays by means of the multiple total external reflections of x-rays from the smooth inner walls of the capillary channels. Reflection of the x-ray photons occurs at the boundary between media with different refractive indices. When an x-ray strikes the reflecting surface of a capillary at a grazing angle greater than the critical angle of the material, it undergoes total external reflection. X-rays satisfying the total reflection condition can be effectively transported through the capillary channels.
Polycapillary X-ray optics formed from millions hollow glass channels bundled and fused together and tapered to desired profiles have been used to control x-ray beams for many varied applications. One of the distinguishing features of polycapillary optics is their broad energy (wavelength) bandwidth.
U.S. Pat. Document No. 2004000062347, issued Apr. 1, 2004 relates to X-ray microscope in which the placement of the test object is located between extended X-ray source and lesser end side of the X-ray capillary lens and the resolution is fully determined by dimension of the channel entrance of the capillary lens.
Fiber Optic Scintillating Plates
Fiber optic scintillating plates or fiber optic scintillators are structurally identical to standard fiber optics and formed from an array of scintillating glass fibers disposed substantially parallel to one another. The core of each of the glass fibers is doped with a scintillating material such as terbium to emit visible light when exposed to x-rays, UV light or ionizing particles. The surface of each of the glass fibers is clad with a non-scintillating, lower optical index glass material which is essential in minimizing cross-talk between fibers. Therefore the produced visible light as a result of x-ray absorption within each fiber core will be channeled and directed toward the imaging sensor, such as a CCD. To capture more of this light, the input side of the plate is usually coated with a reflective material, such as aluminum. This has the effect of re-directing that portion of the light which propagates back toward the input face. The fiber optic scintillating plates can produce images with very high resolution due to the discrete and channelized nature of the emission of light within the core of each fiber. The thickness of fused faceplates is a function of the energy of the radiation to be converted. Fiber optic scintillators are generally suitable for medical imaging, for example, using x-rays having energies of about 50 KeV to about 80 KeV.
U.S. Pat. No. 5,391,320 Buchanan, et al. February, 1995, relates terbium activated silicate luminescent glasses. Terbium glass fiber optic scintillators offer an easy solution for X-ray and γ-ray detectors at higher energy. (10 kV Upwards) The conversion efficiency is lower at around 10 photons per keV, but since all the light is channeled down the fibers, the thickness can be made whatever is necessary to achieve efficient absorption.
U.S. Pat. No. 5,554,850, issued September, 1996 relates to X-ray scintillating plate utilizing angled fiber optic rods.
U.S. Pat. No. 6,384,400 issued May, 2002 relates to a high resolution and high luminance scintillator. In this patent, a fiber optic scintillator having a plurality of double clad radiation absorbing fibers is described. Each of radiation absorbing fibers includes an inner scintillating fiber surrounded by an outer radiation absorbing clad. Inner scintillating fiber desirably comprises a scintillating glass fiber core with a lower optical index glass clad to increase the critical angle for internal reflection of light. Outer radiation absorbing clad desirably comprises a high-density glass. The initial double clad radiation absorbing fibers are formed, for example, by inserting the scintillating fiber inside of a lower optical index glass tube and then inserting the assembly into a larger high-density glass tube. This assembly is then heated and drawn down to a smaller diameter either before or after assembling into a fiber bundle.
Night Vision Optic Image Intensifier
Night vision devices were developed for military use to enhance our night vision. The device includes an objective lens which focuses invisible infrared light from the night-time scene through the transparent window member, a vacuum chamber carrying a photocathode behind the transparent window member, a micro channel plate (MCP), a phosphor screen on the inner surface of a fiber optic faceplate and a high-voltage power supply. A visible image on phosphor screen which is deposited on the image output window of the out surface of the fiber optic faceplate is then presented via an eyepiece lens to a user of the device as shown in FIG. 6.
The device both amplifies the image from the scene and shifts the wavelength of the image into the portion of the spectrum which is visible to humans, thus to provide a visible image replicating the scene.
The photocathode is responsive to photons of visible and infrared light of an image of a night-time scene to liberate photoelectrons which are moved by a prevailing electrostatic field to a micro channel plate causing a geometric cascade of secondary-emission electrons moving along the micro channels, from one face of the micro channel plate to the other so that a spatial output pattern of electrons (which replicates the input pattern; but at an electron density which may be, for example, from one to several orders of magnitude higher) issues from the micro channel plate. This pattern of electrons is moved from the micro channel plate to a phosphorescent screen by another electrostatic field. When the electron shower from the micro channel plate impacts on and is absorbed by the phosphorescent screen electrode, visible-light phosphorescence occurs in a pattern which replicates the image.
The necessary electrostatic fields for operation of a night vision device are provided by a high voltage electronic power supply.
Proximity-focused intensifiers are free from geometrical distortion or shading because the photoelectrons follow short, direct paths between the cathode, output screen, and the MCP rather than being focused by electrodes. The overall photon gain of these devices averages about 10,000, which is calculated according to the equation:Gain=QE×G(mcp)×V(p)×E(p)where QE is the photocathode quantum efficiency (0.1 to 0.5 electrons/photon), G(mcp) is the micro channel plate gain (averaging between 500–1000), V(p) is the voltage between the MCP and the output phosphor (around 2500–5000 volts), and E(p) is the electron-to-light conversion efficiency of the phosphor (0.08–0.2 photons/electron). The gain of the micro-channel plate is adjustable over a wide range with a typical maximum of about 80,000 (a detected photon at the input leads to a pulse of 80,000 photons from the phosphor screen).
As shown in FIG. 7, photons in the energy range of 10 eV–1 keV (VUV) are strongly attenuated by air, but can be imaged in vacuum by phosphor screens, or micro channel plates. The detection efficiency of a micro channel plate for photons is a function of incidence and photon energy. Typical efficiency is 10% falling away at both low energy and high energy. The efficiency can be increased to around 20% by coatings such as Csl, but the thickness of this should be optimized for the desired photon energy. Photon energy in 1–50 kV can be imaged by inorganic powder scintillators providing 20–30 visible photons per absorbed kV. At high photon energies the thickness of the scintillator required for good absorption efficiency increases.
The time resolution of image intensifiers and micro channel plates is largely dependent on the readout system.
Other examples of electronic transducers or image capture devices that may be utilized include CMOS image sensors, and other detectors (such as ferroelectric detectors) which provide an electronic signal in response to an electron flux.
Different photocathode quantum efficiencies are shown in FIG. 8. The latest generation of image intensifiers (denoted blue-plus Gen III or sometimes Gen IV) employs smaller microchannels (6 micron diameter) and better packing geometry than in previous models with a resultant substantial increase in resolution. The broad spectral sensitivity and high quantum efficiency of the “high blue” GaAs and gallium arsenide phosphide (GaAsP) photocathodes are ideally suited to applications in fluorescence or low-light-level microscopy.
3. Some Drawbacks and Needs
A lot of references clearly indicate that many efforts have been made to develop a subwavelength to nanometer spatial resolution, high time resolution, high image contrast microscope that is relatively easy to operate, less expensive, no focusing, requires little or no specimen preparation, and is relatively portable and small enough for use in the field, provides an enhanced field of direct-view, low distortion brighter images, minimizing or avoiding the need for raster scanning, including several of the attractive features of optical microscopy, such as nondestructiveness, low cost, high speed, reliability, versatility, accessibility, and informative contrast. This invention is one of the efforts among them and is concerning the following points which need to be developed or improved.
Spherical Aberration, Chromatic Aberration and Astigmatism
Fiber-optic tapers do not exhibit any of the so-called Seidel aberrations of lenses, such as spherical aberration, chromatic aberration, coma, or astigmatism. When properly made, they exhibit no significant distortion.
It is need to develop an optical microscope that applies fiber-optical taper instead of lens as the image detector to avoid the image aberrations mentioned above.
Zero Working Distance
The distances between the sample and the detector of traditional optical lens equipped microscopes are much larger than the wavelength of the light source. They are based on far-field optics. Scanning probe microscopes such as scanning tunneling microscopy, atomic force microscopy and near-field scanning optical microscopy, working by measuring a local property—such as height, optical absorption, or magnetism—with a probe or “tip” placed very close to the sample, are near-field microscopes. Both of these two kinds of microscopes can not decrease the working distance to zero and therefore can not avoid the interference of the medium between the sample and the image detector.
As shown in FIG. 9, traditional projection roentgenoscopy is a contact-field imaging method. In such methods and devices, the visible image of the object's internal structure, for example, tissues of a biological object, is obtained as a shadow projection. Density of the acquired image in each of its points is determined by the total attenuation of X-rays that passed through the object on their way from the source to the detection means such as a fluorescent screen or an X-ray film, which is held in contact with the object.
The contact imaging method does not use any far-field enlarging optical system and hence does not cause any aberration and the image of the specimen is blurred scarcely because the specimen is held in contact with the fluorescent screen. Thus, in principle, the contact imaging method is able to form an image of high resolution. The resolution achievable by the contact imaging method is dependent on the particle size of the fluorescent screen. The contact imaging method is able to form images of a very high resolution if the fluorescent screen resolution is high enough.
Japan Pat. 3,573,725, issued February 2003, and U.S. Pat. Document No 20040005026 A1 issued January 2004, relate to X-ray microscope apparatus. The X-ray microscope apparatus (as shown in FIG. 10) holds a specimen on a photocathode in close contact condition, and irradiates the specimen from behind with X-rays generated by the X-ray generator to form an electron image of the specimen by X-rays penetrated the specimen on the photocathode. Then, the electron image enlarging device pulls electrons emitted by the electron image to accelerate the electrons for travel in a direction opposite a direction toward the X-ray generator, and forms an enlarged electron image on the surface of the electron beam detecting device. The image processing device processes the electron image formed on the surface of the electron beam detecting device to display a visible image. Parts of the photocathode irradiated with incident X-rays emit amounts of photoelectrons according to the intensities of the incident X-rays fallen thereon, respectively, to form an electron image corresponding to the X-ray image.
As shown in FIG. 10, the photocathode is attached to the inner surface of an entrance window, which is covered with an X-ray transmitting film. The sample is held by the outer surface of the entrance window. It is clear that the sample and photocathode are separated by entrance window. They are not in contact with each other and the working distance is not zero.
U.S. Pat. No. 5,045,696, issued Sep. 3, 1991 relates to a photoelectron microscope in which a specimen holder comprises a support layer on one surface of which said specimen is held in contact therewith and a photocathode in contact with the opposite surface of said support layer, and wherein said chamber is provided with a window which is composed of said specimen holder, with said specimen exposed outside chamber and said photocathode disposed inside said chamber. a specimen holder, which comprises a support layer, on the upper surface of which is the specimen in close contact therewith, and a photocathode layer attached to the opposite surface of the support layer. The support layer can be a membrane made of Si3 N4 and having a thickness of the order of 1000 ANG. Again, the specimen and photocathode are separated by a specimen holder and its thickness is about 1000 ANG.
There is further a need for the optical microscope that can work truly at zero working distance to enhance the image spatial resolution and contrast and to expel the interference of medium which exists between sample and image detector when the working distance does not equal zero.
Optical Taper Image Contrast Transfer
Optical taper high quality image contrast transfer is very important for the fiber optical taper as an image magnifier. There are two critical factors which will strongly influence the quality of optical taper image contrast transfer.
The first criterion is that the cladding areas have a smaller index of refraction than the core areas in order for total internal reflection to take place within the core areas. The portion of the light emitted at an angle less than the optical critical angle for the fiber exits the side of the glass fiber and thus may impinge on and be detected elsewhere by the light sensitive imaging sensor. This latter portion of the light or so called fiber-to-fiber cross-talk results in image quality degradation. The high index-contrast optical fiber decreases the extent of fiber-to-fiber cross-talk and provides enhanced radial confinement of an optical signal in the fiber core.
The second criterion is the non-image forming light in the cladding. Loss in contrast transfer through the taper is primarily due to the percentage of non-image-forming light transmitted by the cladding of the taper. The cladding material occupies about 25–30% of the commonly high resolution taper's face area. This large amount of non-imaging (scattered) light limits the ratio of contrast transferred through the taper. Further increase in contrast can be achieved by a method called end blocking in which the clad portion at the large face is removed and replaced by a black material. If the taper design contains extramural absorber (EMA), the stray light through the cladding can be eliminated. The EMA is provided by incorporation of small black glass rods between the clear clad fibers. These absorbing rods, although covering only a small fraction of the cladding area, are able to absorb almost all the stray light in the cladding, leading to a substantial reduction in light scatter in the taper and thus significantly increasing the contrast transfer. If the cladding area is a light blocking material, such as a black matrix material, then the additional benefit of improvements in the on-axis contrast can be obtained.
U.S. Pat. No. 6,801,698, issued Oct. 5, 2004 relates to high index-contrast fiber waveguides and the materials for forming high index-contrast fiber waveguides. The optical fibers that utilize total internal reflection to confine light to a core can provide enhanced radial confinement of an optical signal in the fiber waveguide. The enhanced radial confinement can reduce radiative losses, thereby improving transmission efficiency. Moreover, in addition to enhanced radial confinement, it is also possible to achieve enhanced axial confinement in the fiber waveguide. The fiber core has a refractive index more than 2.5 (such as 2.8) and the fiber cladding has a refractive index less than 1.45 (such as 1.4). The absolute difference between the refractive indices of the fiber core and cladding is more than 1.2 (such as 1.4).
A tapered coherent fiber bundle imaging device for near-field optical microscopy is described in U.S. Pat. No. 6,016,376 by Ghaemi et al., in which a subwavelength-resolution optical imaging device is provided. The device comprises a coherent fiber bundle and each optical fiber comprises a core having an index of refraction n1 and a cladding having an index of refraction n2 thereby providing a value N.A., where:       N    .    A    .    =                              N          1          2                -                  N          2          2                      .  Light is effectively confined within the core of each fiber without need for a separate coating applied to the fibers by selecting the values of n1 and n2 for each optical fiber so as to provide a value for N.A. which permits at least a predetermined fraction of the light launched into each optical fiber at the sampling end of the coherent fiber bundle to be conveyed through the optical fiber to the observation end of the coherent fiber bundle, and which restricts crosstalk between adjacent optical fibers of the coherent fiber bundle to a predetermined level.
For a high magnifying fiber optical taper, the methods mentioned above is not simple enough and effective enough to confine the image light in optical fiber core and to block all of non-image forming light in the cladding totally. It is still need to provide a simple and effective method by which an optical taper is formed by high index-contrast fiber with an extra-radiation absorbing cladding to eliminate all of the stray light in the cladding.
Medium Interference Between Sample and Detector
Macroscopical phenomenon of nature is strongly depending upon her microscopic state of energy. The energy variation of life relates to low level energy photon, especially infrared radiation.
The measurement of infrared radiation is complicated by the fact that water and bodily fluids are opaque to infrared light. Consequently, even the slightest amount of water, bodily fluids, moisture, CO2 and many other medium on the collection end of a probe impairs the collection of infrared light. As a result, conventional far-field and near-field optics is not good enough to be used in infrared procedures where moisture or bodily fluids are present.
In an article by Lewis A., et al entitled “Near-field optics: from subwavelength illumination to nanometric shadowing,” Nature Biotechnology 21, 1378–1386 (2003) Lewis A. et al described: “Probably the most exciting application of this sort of external illumination protocol is the imaging of chemical alterations in a sample by monitoring the scattering of infrared radiation within the region of the electromagnetic spectrum where vibrational modes of surface molecules absorb light in chemically specific ways. Such chemical identification with high spatial resolution is very important for numerous areas of interest in biology. These extend from the chemical identification of molecular entities on biochips to the spatially resolved nanometric imaging of highly compartmentalized cell membranes. Of course, application of this latter methodology to biological imaging is subject to the problem of high absorption of infrared radiation by water”.
There is indeed a need to provide a microscope which can work at zero working distance to expel the interference of the medium between sample and detector and to obtain a dynamic functional imaging with high spatial and time resolution and with broad-spectrum.
Optical Taper Image Pre-Enlargement
The prior art are exemplified in Japan Pat. 3,573,725, issued February 2003, and U.S. Pat. Document No 20040005026 A1 issued January 2004, entitled X-ray microscope apparatus described that a specimen is held on a photocathode in close contact condition, and irradiates the specimen from behind with X-rays generated by the X-ray generator to form an electron image of the specimen by X-rays penetrated the specimen on the photocathode without any pre-magnification. Then, the electron image enlarging device pulls electrons emitted by the electron image to accelerate the electrons for travel in a direction opposite a direction toward the X-ray generator, and forms an enlarged electron image on the surface of the electron beam detecting device.
In principle, the contact imaging method without any pre-magnification is able to form images of a very high resolution if the resolution of photocathode and micro-channel plate is high enough. The resolution achievable by the contact imaging method without any pre-magnification is dependent on the photocathode and micro-channel plate spatial resolution which is not high enough for the microscope need.
It is necessary to provide a microscope with optical taper as image pre-magnifier to pre-enlarger the sample image in order to match the limited resolution of photocathode and micro-channel plate.
Spatial and Time Resolution
The resolving power of a microscope is one the most important feature of the optical system and influences the ability to distinguish between fine details of a particular specimen. The demand for the dynamic information focused on the physics and chemistry of microstructures at submicron and nanometer scale range to challenge in microscopy and spectroscopy today is not only the improvement of finer and finer resolution, but also the development of techniques for observing sample events in real time, as they happen, without destroying the sample in the process. However, conventional scanning probe microscopes generate images of sample areas slowly due to the sequential imaging of small, discrete points of sample areas required by the raster scanning technique.
There is further a need for an optical microscope that provides high resolution real-time image in normal atmosphere and temperature and in natural state and environment, without interference from the artifacts of specimen preparation, without destroying or altering sensitive biochemical characteristics, and without disturbing the specimen.
Combination of Optical and Electron Enlargement
Electron image has short wavelength, high resolution, no medium (such as water, air et al) interference in vacuum condition and is easy to be magnified by electromagnetic lens. The magnification of the electron image enlarging device can continuously be varied by adjusting currents supplied to the magnetic lenses. Therefore, a minute object can precisely be located and observed by determining the position of the object using the electron image enlarging device at a low magnification and displaying a desired object at a high magnification. Unlike electron image, the magnification of optical taper image is limited and difficult to adjust.
Although electron microscopes offer very fine resolution, the specimen must be prepared by high-vacuum dehydration and is subjected to intense heat by the electron beam, making observation of living specimens impossible. The dehydration process also alters the specimen, leaving artifacts and sample damage that were not present in nature.
It is real need to develop a microscope that use optical taper as zero-working distance interface to transfer sample optical image into vacuum chamber to combine the advantages of high resolution, high magnification characters of electron microscope and to avoid keeping sample in vacuum condition.
Photon Input Flux Density Adjustable Intensifying
The overall photon gain of image intensifier averages about 10,000, which is calculated according to the equation:Gain=QE×G(mcp)×V(p)×E(p)where QE is the photocathode quantum efficiency (0.1 to 0.5 electrons/photon), G(mcp) is the micro channel plate gain (averaging between 500–1000), V(p) is the voltage between the MCP and the output phosphor (around 2500–5000 volts), and E(p) is the electron-to-light conversion efficiency of the phosphor (0.08–0.2 photons/electron).
The gain of the micro-channel plate is adjustable over a wide range with a typical maximum of about 80,000 (a detected photon at the input leads to a pulse of 80,000 photons from the phosphor screen).
It is useful to provide a microscope that can adjust photon input flux density by changing the gain of the micro-channel plate to satisfy different irradiation of the sample.
Fiber Optic Scintillating Taper
Fiber optic scintillating plates or fiber optic scintillators are structurally identical to standard fiber optics and formed from an array of scintillating glass fibers disposed substantially parallel to one another. The core of each of the glass fibers is doped with a scintillating material such as terbium to emit visible light when exposed to x-rays, UV light or ionizing particles. The surface of each of the glass fibers is clad with a non-scintillating, lower optical index glass material which is essential in minimizing cross-talk between fibers. Therefore the produced visible light as a result of x-ray absorption within each fiber core will be channeled and directed toward the imaging sensor, such as a CCD.
To capture more of this light, the input side of the plate is usually coated with a reflective material, such as aluminum. This has the effect of re-directing that portion of the light which propagates back toward the input face.
The fiber optic scintillating plates can produce images with very high resolution due to the discrete and channelized nature of the emission of light within the core of each fiber. The thickness of fused faceplates is a function of the energy of the radiation to be converted.
The fiber optic scintillating plates described above can work as contact-field model, but can not act as a sample image magnifier and require the coating with a reflective material, such as aluminum on the input side of the plate to re-directing the portion of the light which propagates back toward the input face.
There is a need to develop a fiber optic scintillating taper which does not need to coat a reflective material on the input side of the taper and can not only work as contact-field model but also can work as sample image magnifier. The microscope with this fiber optic scintillating taper can invert γ-ray, x-rays, UV light or ionizing particle sample image to an enlarged visible light image.